This brings classification in line with phylogeny (how living things evolved). In cladistics, clades are the only acceptable units.

Cladogram (family tree) of a biological group. The red and blue boxes represent clades, i.e. complete branches. The green box is not a clade, but rather represent an evolutionary grade, an incomplete group: the blue clade descends from the same ancestor but is not included in it.

Some versions of cladistics have been the subject of controversy.[3]p226[4][5]

The term clade was introduced in 1958 by Julian Huxley, cladistic by Cain and Harrison in 1960, and cladist (for an adherent of Hennig's school) by Mayr in 1965.[6] Hennig referred to his own approach as phylogenetic systematics.[1] From the time of his original formulation until the end of the 1980s cladistics remained a minority approach to classification.

For some decades in the mid to late twentieth century, a commonly used methodology was numerical taxonomy.[3]p221 This made no attempt to resolve phylogeny, only similarities. The weakness of this approach was that it left out the connection between classification and evolution.[5]

Phylogenetic nomenclature is a way of giving names to the groups (clades) that have been decided on by cladistic methods. It differs in many ways from Linnaean nomenclature.[8] Critics of phylogenetic nomenclature include Ashlock,[9] Mayr,[10] and Williams.[11]

Phylogenetic Nomenclature

Linnaean Nomenclature

Handles arbitrarily deep trees.

Biased towards trees about 4 to 12 levels deep.

Primary goal is to reflect the process of evolution, as currently understood

Primary goal is to group organisms in a clear and useful way

Assumes that the shape of the tree will change frequently with new discoveries

New discoveries may require releveling of Genera, Classes, Orders, and Kingdoms